1. Field of the Invention
This invention relates to devices for raising the elevation of a manhole cover, typically required prior to or after a street has been resurfaced.
2. Discussion of the Prior Art
Manholes are often constructed from a pre-cast concrete or brick and mortar chimney leading from the sanitary sewer up to about a foot beneath the surface of the roadway. On top of the chimney is placed a cast iron or steel frame, which defines the inlet to the manhole at the street level. A ledge is formed at the inlet on which rests the manhole cover.
When a street is resurfaced, the new pavement surface is usually about an inch or two above the top of the preexisting manhole frame, depending upon the thickness of the new surface. To bring the manhole cover up to the new level of the roadway, it is known to place an adjusting ring on the manhole ledge of a height equal to the new layer of pavement. The old cover is then placed inside the adjusting ring, which provides a new, higher support ledge for the cover.
One known way of securing an adjusting ring to a manhole frame is to provide a gap in the ring and weld a V-shaped expansion lock to the ends of the ring and bridging the gap. With this type of device, the adjusting ring is dropped into the old frame and the V-shaped expansion lock is hammered to flatten out the V into a locked position. This expands the ring tightly against the old frame, but only if the ring was properly sized relative to the frame in the first place. These rings therefore had to be accurately sized or they would end up too tight or too loose when the "V" was flattened or attempted to be flattened. Moreover, since manhole frame sizes are not universally standardized, these types of rings had to be carefully custom fit to specific frames.
Another type of manhole grade adjusting ring was split at one point and a "toggle" mechanism, made up of a pair of pivotally mounted bolts which could be rotated past center, bridged the split and could be hammered into place to expand the ring. This type of adjusting ring is described in U.S. Pat. No. 4,097,171. The bolts were threaded so that they could first be adjusted to the appropriate length needed to enable the mechanism to be toggled past center by hammering to expand the ring and thereby secure it to the manhole frame.
As the art developed, problems with this type of mechanism became apparent. For example, U.S. Pat. No. 4,225,226 states that such mechanisms were too expensive to manufacture and install (column 1, line 59-column 2, line 4). U.S. Pat. No. 4,225,266 also discusses (column 1, lines 36-51) that the threads could become corroded or could become loosened because of vibration caused by passing vehicles. U.S. Pat. No. 4,302,126 expands upon the problems with the device of 4,097,171, stating that it could become damaged during storage and shipment since the links flapped loosely on their pivotal mounting at such times, that the range of hole sizes was somewhat limited for the 4,097,171 device and that the toggle links could sometimes form too sharp of an angle in the disengaged position, making it extremely difficult to hammer the links past center to secure the adjusting ring.